Generally, electrical rotating machinery such as generators and motors use slip rings and brushes to transfer current to and receive current from rotor windings mounted on a rotor. For example, FIG. 1 illustrates an embodiment of a rotor 102 having a slip ring 104 mounted on or attached to the rotor 102. An electrical brush 106 is in contact with the slip ring 104. Electrical current passes through the electrical brush 106 into the slip ring 104 or from the slip ring 104 into the electrical brush 106. The electrical brush 106 is typically held in position by a brush holder (not shown). Also, typically one or more springs (not shown) are used to apply pressure to the electrical brush so that contact between the brush 106 and the slip ring 104 is continuous as the slip ring 104 is rotated by the rotor. Typically, the electrical brush 106 is at least partially comprised of conductive carbon graphite. This material tends to wear over time as the slip ring 104 rotates under the electrical brush 106. The wear of the brush 106 depends on many parameters as e.g. temperature, humidity, eccentricity of the rotor, contact force, speed of the rotor, etc. Information about the influence of different parameters on brush wear can be found in, for example, M. Wuerfel and W. Hofmann, “Monitoring and Diagnostics of the Rotor Slip Ring System of Three Phase Current Induction Generators,” TU-Chemnitz, 2002; M. Bryant, A. Tewari and J-A Lin, “Wear Rate Reductions in Carbon Brushes, Conducting Current, and Sliding Against Wavy Copper Surfaces,” IEE Transactions on Components, Packaging and Manufacturing Technology-Part A, Vol. 18, No. 2, June 1995; among others. Due to the large variety of parameters which have an influence on the wear of the brushes 106, it is not easy to determine their remaining lifetime. This wear can eventually result in failure of the brush 106 or the slip ring 104 if these components are not maintained. As the electrical brush 106 typically wears faster than the slip ring 104, maintenance usually involves replacement of the electrical brush 106. Electrical brushes 106 are designed with this wear in mind. Therefore, they can wear down a certain extent before replacement is required. In some instances, though, maintenance is performed on a time basis, not on the basis of the condition of the brush 106. This may result in costly down-time of the electrical rotating machinery that may not be required. In other instances, the brush 106 may be fitted with a micro-switch that triggers when the brush 106 is worn to a predetermined extent; however, this prevents planned and scheduled maintenance of the brush 106 as it will require replacement when the switch trips.
In some instances, the electrical rotating machinery utilizing the brush 106 and slip ring 104 arrangement can be a generator such but not limited to as a doubly-fed induction generator (DFIG) used in a wind turbine. As described herein, the micro-switch reduces the risk of catastrophic failures to the generator and therefore to the whole wind turbine. However, it still causes unplanned maintenance which is more expensive than scheduled maintenance and therefore not desired. In relation to the growing number of wind turbines under full service agreements (FSA) the implementation of an improved brush wear monitoring system is desired.
Therefore, systems and methods are desired that overcome challenges in the art, some of which are described above.